how to 09 o choose an intensive care unit ventilator pelosi 2008

94 THE BUYERS’ GUIDE TO RESPIRATORY CARE PRODUCTS

HOW TO CHOOSE AN INTENSIVE CARE UNIT VENTILATOR09

BG-09 Ventilators:BG-09 Ventilators 20/8/08 23:13 Page 94

The purpose of this guide

A range of ventilators is available tothe intensive care unit (ICU)physician, something that may intheory enable him or her to treatdifferent forms of respiratory failuremore effectively. New ventilatorymodes are frequently introduced,but could this novelty sometimes beconsidered no more than amarketing tool?

Although these new modes mightto some extent assist the clinician indaily practice, scientific evidenceproving their effectiveness is oftenlacking [1]. When purchasing aventilator, clinicians rely onpersonal experience and on theresults of small observational trialsshowing positive effects onphysiological variables, such asoxygenation and work of breathing;and on subjective variables, such aspatient comfort [1].

Ideally, the process of purchasing aventilator should be based on astrong scientific rationale [2, 3],founded on predeterminedrequisites and scores [4, 5]. Becausemanufacturers’ specifications aloneare of limited importance, theyshould not be a prominent factor indecision making. A basicknowledge of the principles of

ventilator functioning may behelpful when choosing amechanical ventilator, enabling thepurchaser to consider its technicalperformance in relation to theclinical characteristics of thepatients to be treated, the healthcareenvironment and the financialresources available [6, 7].

The primary purpose of this guideis not to compare the individualfeatures of each ventilator, butrather to provide information abouttheir technical aspects, which mayassist in the purchasing decision [6].

Basic principles of ventilatorfunction

Briefly, a mechanical ventilator canbe considered as a series ofconsecutive functions that turn aninput (energy) into an output(ventilatory variable), such aspressure, flow or volume. It cantransfer energy by applyingpositive pressure to the airways,acting as a positive pressureventilator (PPV), or by applyingsubatmospheric pressure externallyto the chest, acting as a negativepressure ventilator. This article willfocus solely on PPVs. There areseveral fundamental elements to aPPV [6, 8], as shown in table 1.

1. Pneumatic system

All ICU PPVs, except for portabletransport ventilators that may bedriven pneumatically, requireelectricity (AC external power or aDC internal battery).

The gas source can be:

a) an external high-pressure gas(centralised gas system or tanks);

b) an internal compressor;

THE BUYERS’ GUIDE TO RESPIRATORY CARE PRODUCTS 95

HOW TO CHOOSE AN INTENSIVE CARE UNITVENTILATOR C. Gregoretti, P. Navalesi, I. Tosetti and P. Pelosi

Correspondence

C. GregorettiDipartimento Emergenza AccettazioneASO CTO-CRF-Maria AdelaideVia Zuretti 2910129 TorinoItalyFax: 39 116933266

E-mail: [email protected]

HOW TO CHOOSE AN INTENSIVE CARE UNIT VENTILATOR 09

1. Pneumatic system

2. Inspiratory and expiratoryvalves and output variablecontrol

3. Phase variables: trigger, cyclingoff and limit

4. Modes of mechanical ventilation

5. Control system

6. User interface

7. Safety and alarm systems

8. Monitoring system

9. Respiratory circuit andaccessories

Table 1 Fundamental elements of a positivepressure ventilator


c) a turbine or piston; or

d) a combination of (a +c) or (a+b).

Based on the gas source, ICU PPVscan practically be divided into twocategories: those that work withoxygen and air at high pressure (4 atm (400 kPa)); and those thatwork with oxygen at high pressure (4 atm (400 kPa)) and atmospheric air.

Category 1. Oxygen and air at highpressure PPVs

High-pressure air and oxygen arethe external source of energy forthis type of ventilator (figs 1–8).

Pressure inside the ventilator isthen reduced to atmosphericpressure by a “pressure reducerdevice” to allow the patient toinhale. While some old ICU PPVsallowed the physician tomanipulate the working (driving)pressure, for safety reasons none ofthe current generation of PPVsallow this [6]. The peak flow outputof these PPVs is usually up to 200 L·min-1 (with constant flowabout 130–140 L·min-1) with a veryfast pressure rise time (pressure in agiven time with flow as thedependent variable) when usingassisted pressure-limited, time-cycled (e.g. assisted pressure-

controlled ventilation (AC/PCV)) orflow-cycled ventilatory support (e.g.pressure support ventilation (PSV))[9]. Pressure rise time may be



Percentage of pressure slope

Percentage or a given value offlow#

Steepness of pressure rampgradient

Pictograms depicting differentpressure ramps#: In this case, pressure slope changesaccording to flow setting.

Table 2 Ways to set pressure rise time

Figure 3. TaemaExtend category 1ventilator

Figure 4. Maquet Servo i category 1 ventilator

Figure 5. Covidien PB 840category 1 ventilator

Figure 6. Dräger Evita XLcategory 1 ventilator

Figure 7. GE EngstromCarestation category 1 ventilator

Figure 8. Viasys Avea category 1ventilator (Cardinal Health)

Figure 1.eVentInspirationLS category 1ventilator

Figure 2. Hamilton MedicalGalileo, G5 and Raphael XTCcategory 1 ventilator


PRODUCT LISTINGS



regulated in most of theseventilators and its setting may havea real clinical application [6, 10, 11].Table 2 shows the methods ofdefine pressure rise time.

Category 2. Oxygen at high pressure(4 atm) plus atmospheric air PPVs

In these ventilators, a piston orturbine sucks atmospheric air fromthe environment. Although piston-or turbine-driven ventilators areusually used for home-care positivepressure ventilation [12], thesimultaneous use of high-pressureoxygen and a sophisticated userinterface make these PPVs suitablefor critical care use. In the past, amajor problem with turbine-drivenventilators was that in spite of avery high peak flow output (up to240 L·min-1) in basal conditions(without any additional resistance),the application of a given resistancesignificantly decreased theventilatory output (on occasion to<100 L·min-1). In addition, duringvolume-targeted ventilation, it wasdifficult to maintain a given tidal

volume when facing increasedpatient elastic or resistive load.

Fast turbines (“dynamic blowersystems”; e.g. Viasys Vela and ViasysPulmonetic LTV 1200 (both CardinalHealth), ResMed Elisée 350,VersaMed iVent 201 IC, DrägerCarina) or turbines rotating atconstant speed (“constant-revolutionblower systems”) driven by aproportional valve (e.g. DrägerSavina, Respironics Esprit,Respironics Vision, Taema NeftisICU, Hamilton Medical C2) make thelatest generation of turbine-drivenPPVs (figs 9–19) as efficient as thosedriven by high-pressure gas [9].

From the standpoint of highresponsiveness to patient’s flowdemand, constant-revolutionblower systems with aproportional valve perform verywell. However, although in thepast these systems had a clearresponsiveness advantage overdynamic blower systems (whichchange speed to reach the presetventilatory ouput), recent

developments show that dynamicblowers with a small blower wheeldiameter and a very highrevolution rate per minute are alsoextremely responsive to patientdemand [13]. Some turbine-drivenventilators use a bleed oxygen flowto increase responsiveness;however, this has the disadvantageof high oxygen consumption.

In this ventilator category, too,pressure rise time can be adjusted.However, the clinical significance ofthis may be different from that incategory 1 ventilators, becausepressure rise time is critical at thevery beginning of inspiration (0.3 s),particularly in dynamic blowersystems [9].

Gas blending

Ventilators no longer incorporateaccumulators, as for instance in theold Servo 900 [6]. Instead, allcategory 1 and some category 2ventilators (e.g. Dräger Savina,Respironics Esprit, RespironicsVision) blend gases using an internal


HOW TO CHOOSE AN INTENSIVE CARE UNIT VENTILATOR


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blender driven by a proportionalvalve. Some PPVs use a proportionalsolenoid valve (e.g. Covidien PB 840,Respironics Esprit). The presetoxygen tension is constant, whateverthe minute volume. Most category 2PPVs do not have a real blender:oxygen is delivered by aproportional valve and mixed withair coming from the turbine. Again,the preset oxygen tension is constantregardless of the minute volume.

Some category 2 ventilators can alsohave a low-pressure oxygen inlet(e.g. VersaMed iVent 201 AB/IC,

Viasys Vela and Viasys PulmoneticLTV 1200, ResMed Elisée 350,Dräger Carina and Savina, TaemaNeftis ICU, Hamilton Medical C2).In spite of the Fi,O2 that an internaloxygen sensor may read, oxygendelivery is not constant and Fi,O2

cannot reach 100%.

Although helium–oxygen breathingis known to reduce airwayresistance and may therefore beindicated in conditions such asacute asthma, acute exacerbationsof chronic obstructive respiratorydisease or acute upper airway

obstruction, so far its clinical use islimited, in part because of the lackof adequate ventilators. As a furtherrecent development, somecommercially available ICUventilators now incorporate thetechnology for helium–oxygenventilation (e.g. Viasys Avea(Cardinal Health), Maquet Servo i,eVent Inspiration LS) [6, 14].

Internal battery

Most category 1 PPVs have aninternal battery, usually with a

Figure 9. Taema NeftisICU category 2ventilator

Figure 12. RespironicsEsprit category 2ventilator

Figure 13. ViasysPulmonetic LTV1200 category 2ventilator(Cardinal Health)

Figure 14.Hamilton MedicalC2 category 2ventilator

Figure 15. ResMedElisée 350 category 2ventilator

Figure 16. VersaMediVent 201 category 2ventilator

Figure 17. DrägerCarina category 2ventilator

Figure 18. CovidienAirox Supportaircategory 2 ventilator

Figure 19.Newport

HT50category 2ventilator

Figure 10. Dräger Savina category 2ventilator

Figure 11. Viasys Vela category 2 ventilator(Cardinal Health)


short life per charge. Interestingly,some category 1 PPVs have a “plugand play” battery system (e.g.Maquet Servo i, Dräger Evita XL,Hamilton Medical G5) to increasebattery life. Some category 1ventilators (e.g. eVent InspirationLS, Viasys Avea) have an internalbattery that can power an internalcompressor.

Some PPVs, especially those ofcategory 2, have a very long-lasting(4–9 h) internal battery (e.g.Covidien Airox Supportair, ResMedElisée 350, Viasys Vela). Others maybe equipped with a supplementalbattery providing energy for up to4–7 h (e.g. Dräger Savina, VersaMediVent 201 IC, Hamilton Medical C2).This can be very useful whentransporting critical patients insidethe hospital.

Recommendations to the buyer

1. The buying team must be awareof the differences in terms of gasinput and type of control systemwhen choosing a crtical careventilator. In areas wherecompressed air is not available, aturbine-driven ventilator withadditional oxygen (category 2)may replace compressed air.

2. In all category 2 ventilators thatalso have a high-pressure oxygeninlet preset oxygen tensionremains constant regardless of theminute volume.

3. Some turbine-driven ventilatorsalso have the option of a low-pressure oxygen inlet. Whenusing this option, oxygen tensionis never constant.

4. The charge-life of the internalbattery as well as the possibility ofusing a supplemental battery mustbe taken into account in areaswhere mains power is frequentlyinterrupted and where electricalback-up is inconsistent.

5. When choosing category 2 PPVsin areas where oxygen isprovided only in tanks,ventilator oxygen consumptionfor a given minute ventilation

must be taken into account. Insome markets, oxygen may be areal cost issue.

2. Inspiratory and expiratoryvalves and output variablecontrol

The inspiratory valve is meant tocontrol respiratory cycle phases,along with the expiratory valve. Incategory 1 PPVs and some category2 PPVs, the valve manages theoutput of the ventilator (set point,auto-set point, servo, adaptive andoptimal control with controlledproportional valve). For instance,auto-set point control is called:volume-assured pressure support(VAPS) in the Respironics Esprit;AutoFlow in the Dräger Evita 2Dura, 4 and XL and Savina; orAutomode in the Maquet Servo i.Servo control is called proportionalassist ventilation (PAV+) in theCovidien PB 840; adaptive control isknown as pressure-regulatedvolume control (PRVC) in theMaquet Servo i and Taema Extend,or pressure-controlled volumeguaranteed in the GE EngstromCarestation; optimal control iscalled Adaptive Support Ventilation(ASV) in the Hamilton Medical G5[6, 10].

In many category 2 PPVs, theinspiratory valve has only an on-offfunction: pressure and flow bothdepend on the mechanical system(the piston or on the rotationalspeed of the turbine; set pointcontrol without a proportionalvalve) [6, 8].

In category 1 PPVs, theproportional valve (in current-generation models always asolenoid valve) can function aseither a flow- or a pressure-controller. With pressure or flowcontrol, the delivered waveform isa function of both ventilatorsetting and respiratory systemimpedance. In current-generationcategory 1 ventilators and in somecategory 2 ventilators, threewaveforms are available:



Figure 20. Medisize Zephyros category 1ventilator

Figure 21. Respironics Vision category 2ventilator




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rectangular; descending ramp involume-targeted mode; andexponential decay in pressure-targeted mode [6]. Although theclinical usefulness of inspiratorywaveform manipulation is unclear,peak pressure is lower and gasdistribution is improved with adecelerating flow pattern(descending ramp).

The expiratory valve can be asimple valve that is closed incounter-phase with the inspiratoryone (a mushroom or diaphragmvalve), or a proportional-aperturevalve. Usually, expiratory valvesalso control the baseline phase,which can be atmospheric pressureor the positive end-expiratorypressure (PEEP). When baselinepressure is increased to create athreshold resistor (which, unlike aflow resistor, ideally maintainsconstant force regardless of gasflow) the efficiency of theexhalation valve functioning maydecrease, prolonging theexpiratory time constant. The useof a microprocessor-controlledexpiratory valve or anelectromagnetic valve operated byan actuating shaft (as in allHamilton Medical ventilators, forinstance) can modify thisinefficiency by reducing pressurein the exhalation valve toatmospheric pressure early in theexpiratory phase [6, 12].

The use of active expiratory valvesmakes it possible for the patient tobreathe spontaneously during amandatory pressure-controlledbreath in controlled or assistedmandatory breathing.Manufacturers refer to pressure-limited ventilation that allowspatients to breathe spontaneouslythroughout the respiratory cycle byvarious names (e.g. assistedpositive-release ventilation (APRV)as well as BIPAP in the Dräger Evita2 dura, 4 and XL and in the BIPAPSavina; BiLevelTM in the CovidienPB 840 and GE EngstromCarestation; BIVENT in the MaquetServo i; BiPhasic in the Viasys Velaand Avea, etc.) [8].


1. Control of output variables andvalve functioning are veryimportant when choosing a PPV,regardless of its pneumatic system.

2. The use of microprocessor-controlled expiratory valves orelectromagnetic valves operatedby an actuating shaft canreduce exhalation systeminefficiencies

3. Phase variables: trigger,cycling and limit

Three phase variables defineinspiration [6, 8]:

• The trigger that begins inspiration(pressure-, volume-, flow- andtime-dependent).

• The limit that cannot be exceededduring inspiration (pressure,volume, and flow).

• The cycling-off criteria.

Trigger variable

During mechanical ventilation abreath can be initiated by [6, 10,15–17]:

• time (timed mandatory breathsset by the operator);

• pressure (assisted breathpressure-triggered);

• flow (assisted breath flow-triggered);

• volume (assisted breath volume-triggered); or

• diaphragmatic activity (NeurallyAdjusted Ventilatory Assist(NAVA)).

The term “mandatory breath”defines a breath that is initiated andcycled by the ventilator.

During partial ventilatoryassistance (assisted breath), theinspiratory synchronisation system(trigger) detects any patientinspiratory effort and activates amechanical act. The goal of a goodinspiratory trigger is to reduce


as much as possible the durationand intensity of the musculareffort that comes beforemechanical support, whileavoiding autotrigger effects.

While pressure triggering allowsdetection of a pressure drop withinthe circuit (at the airway opening orinside the ventilator) due to thepatient’s inspiratory effort, flowtriggering is achieved with themeasurement of flow by using apneumotachograph at the airwayopening (e.g. Hamilton Medicalventilators, Viasys Pulmonetic LTV1200) or inside the ventilator (mostcategory 1 and category 2ventilators). Some flow triggers workwith the “flow-by system”, whichprovides a continuous bias flow intothe circuit with triggering occurringwhen the difference between theflows entering and exiting therespiratory double-limb circuitsequals the trigger sensitivity [6].

This bias flow may be automaticallyadjusted when increasing flowtrigger sensitivity (e.g. Covidien PB840) or else manually adjusted (e.g.Viasys Avea).

New trigger algorithms also aim atimproving patient–ventilatorinteraction during sudden changesin flow or respiratory rate or in thepresence of air leaks duringnoninvasive ventilation (NIV). Thiscan be achieved with volumetriggers, triggers linked to flowwaveform algorithms (e.g.Respironics Vision), combiningpressure and flow signals in thesame trigger algorithm (e.g. DrägerEvita 2 dura, 4 and XL) or usingboth pressure and flow triggers (e.g.Smart trigger Viasys Avea). Table 3shows the types of available triggers.

Unfortunately, reducing auto-triggering often implies a lowertrigger sensitivity. This isparticularly true when ventilatinguncuffed airways. However, it hasbeen generally suggested that atrigger (independently of itsalgorithm) must have a responsetime <100 ms.

All inspiratory trigger drawbacksmay be overcome by using a neuraltrigger obtained by means of adedicated nasogastric tube with amultiple array of electrodes placedin the distal oesophageal portion(NAVA; e.g. Maquet Servo i)[17, 18].

Cycle variable

A breath can be pressure-, time-,volume- or flow-cycled [8]. A breathin a flow-cycled ventilation mode(PSV, inspiratory positive airwaypressure (IPAP) or assistedspontaneous breathing (ASB) forDräger ventilators) is cycled wheninspiratory flow reaches a giventhreshold value (default at 25% ofpeak flow in most PPVs, butadjustable in others). Theinspiratory flow threshold value,also called “expiratory trigger”,thus controls the inspiration-to-expiration switch in these

modalities [19]. The aim is to detectthe very end of patient inspirationthrough inspiratory flowmeasurement. Its goal is to optimisesynchronisation betweenspontaneous patient inspiratorytime and ventilator inspiratorytime. In a flow-cycled ventilatorymodality there is often a rankinglogic of expiratory cycling criteriathat allow cycling to expiration.Table 4 shows criteria for PSV/IPAPcycling to baseline pressure(atmospheric pressure orPEEP/expiratory positive airwaypressure (EPAP)).

Dyssynchrony at the onset andtermination of a PSV breath can becorrected by varying the inspiratoryrate of pressurisation and/or theoff-cycling criteria (e.g. modulatingthe expiratory trigger threshold ) [6,20–24]. All category 1 ventilatorsand most category 2 ventilatorshave this feature.



Pressure trigger where a given negative pressure is set by the operator(e.g. -1 cmH2O)

Genuine flow trigger where a given flow value is set by the operator (e.g. 1 L·min-1)

Flow trigger with predetermined flow-by system, where a given flowvalue is set by the operator#

Flow trigger with adjustable flow-by system, where a given flow value for the trigger and a given flow-by value for the system is set by theoperator¶

Combined genuine flow trigger and predetermined pressure triggerwhere the operator sets only a flow value+

Combined flow-by and pressure trigger where the operator sets pressureand a flow value+

Trigger working with sequential algorithm linked to flow§ or turbinespeed variation. The operator has nothing to set

Volume triggerƒ

Neural trigger (NAVA)##

#: In this case, flow-by increases with decreasing flow sensitivity (e.g. flow-by increases 1.5 L·min-1 for each increment in flow sensitivity (e.g. Covidien PB 840); ¶: In this case patienteffort is detected first by one of the two triggers. There is no ranking logic (e.g. Viasys Avea); +: In this case the pressure trigger acts only as a safety mechanism to avoid auto-triggering (e.g. Dräger Evita 2 dura, 4 and XL and Savina); §: AutoTrak system by Respironics; ƒ: Usuallyadopted with a default volume value (e.g. 20 mL) in some ventilators to avoid auto-triggering;##: Only in Maquet Servo i.

Table 3 Types of inspiratory trigger




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Limit variable

Mechanical ventilators apply positivepressure at the airways with differentmodalities. Although these areincreasingly sophisticated, they areusually divided into volume- orpressure-limited ventilatorysupports. With the former, theindependent variable (ventilatoryoutput) is flow and its integral overtime, namely volume. The dependentvariable is pressure, which variesaccording to the respiratory systemimpedance. With the latter, theindependent variable is pressurewhile flow and volume (dependentvariables) are consequent toventilatory impedance [6, 8].

In some category 2 ventilators (theso-called bilevel ventilators), theset-up governing the peak pressureapplied to the respiratory systemduring pressure-limited flow ortime-cycling ventilatory modes(usually called spontaneous/timed(ST) ventilation with inspiratorypressure, labelled as IPAP, andPEEP labelled as EPAP) may differfrom category 1 ventilators, wherethe same ventilatory modes arecalled PSV/PEEP (spontaneous) orAC/PCV/PEEP (timed).

To give an example, when usingPSV 10 cmH2O plus PEEP 5 cmH2O

in a category 1 ventilator, the actualpressure applied to the patient’sairway above PEEP is 10 cmH2O,while applied peak pressure is 15cmH2O (10 + 5: the so-called “abovePEEP” setting). When using IPAP 10cmH2O plus EPAP 5 cmH2O using abilevel ventilator (e.g. RespironicsVision, Covidien Supportair) theactual pressure delivered to thepatient above EPAP is 5 cmH2O, sothe applied peak pressure remainsat 10 cmH2O (the so-called “belowPEEP” setting). However, mostbilevel ventilators have a reminderon the main screen of the peakpressure applied to the airways.

This fact has generated a lot ofconfusion in the past. To add moreconfusion, a modified algorithm ofassisted positive pressureventilation was named BilevelInspiratory Positive AirwayPressure (BIPAP). This mode hasoften been mistaken for BiPAPTM byRespironics, particularly becauseBIPAP, like all APRV algorithms,uses the below PEEP setting.


1. All commercial PPVs, except onesdedicated mainly to NIV (e.g.Respironics Vision) offer bothpressure- and volume-targetedventilation.

2. In spite of clear evidence that aflow trigger performs better thana well-set pressure trigger [6] thesensitivity of pressure or flowtrigger setting is mandatory toavoid patient–ventilatordyssynchrony.

3. When combining different kindsof inspiratory trigger,manufacturers should specifytheir ranking logic clearly. Oneventilator (Maquet Servo i) offersthe option of using a neuraltrigger through NAVA.

4. Almost all category 1 andcategory 2 PPVs enable the userto set the sensitivity of theexpiratory trigger threshold(expiratory trigger). The widerthe range (e.g. 1–90% of peakexpiratory flow), the higher thechance of matching the patient’sneural inspiratory time.

5. When using turbine-driven orpiston ventilators, the buyer mustbe aware of the set-up governingpeak inspiratory pressure abovePEEP/EPAP during pressure-limited ventilatory modes.

4. Modes of mechanicalventilation

It is beyond the aim of the presentguide to explain all modes ofventilation. The authors will try toinform the reader of those ventilatorymodalities that go beyond thedifference between pressure- andvolume-limited ventilatory support(automatic tube compensation(ATC), PAV, proportional pressuresupport (PPS) and NAVA, whereneither pressure nor volume must beset by the operator). We will alsodeal with those modes of ventilationthat switch from one limit variable toanother one (e.g. from pressure- tovolume-limited ventilatory support,so-called dual modes) and ASV [6, 8].

PAV, PAV+ and PPS

Proportional ventilation was firstdescribed in 1992 [25]. PAV can beviewed as a ventilation mode in

Flow reaches a predetermined percentage of inspiratory peak flow(usually 25%)

Flow reaches a preset percentage of inspiratory peak flow (e.g. from 1% to 80%)

According to particular algorithms linked to flow value or wave form (e.g. Respironics AutoTraK system)

According to a given preset or adjustable inspiratory time (safety cycling)

According to a given default pressure (2–3 cmH2O) above the peakinspiratory pressure (safety cycling)

According to a given default flow value after a given time (usually 1–3 s;safety cycling)

PSV: pressure support ventilation; IPAP: inspiratory positive airway pressure; ASB: assistedspontaneous breathing.

Table 4 Criteria for PSV/IPAP/ASB to cycle to baseline pressure




which the pressure within eachbreath is titrated by the ventilator inproportion to patient inspiratoryflow, which is used as an estimate ofpatient respiratory muscle effort .The proportionality between flowand airway pressure is determinedby a “gain” setting, which is adjustedby the clinician to determine theproportion of the total work ofbreathing to be performed by bothventilator and patient. In contrastwith previous modes of ventilation,in PAV, the clinician sets the gain(proportion of work of breathing,expressed as a percentage) based onpatient respiratory mechanics. Inother words, the resistance (Rrs) andcompliance (Crs) of the respiratorysystem are evaluated and thepercentage of assistance adjustedaccordingly. However, although thedifficulties associated with assessingRrs and Crs in actively breathingpatients are being overcome, (e.g. bythe Covidien PB 840 in PAV+ mode),PAV may become unstable in certainsituations, or if the gain is setimproperly. This drawback of PAVhas been termed “runaway”, andresults when algorithm-inducedchanges affect an input variable,thereby creating an undesirablechange to the output, in a cyclicalfashion (increasing pressure resultsin increased flow, which mayincrease measured Rrs, resulting infurther increases in pressure) [26–29].

NAVA

NAVA is a new form of assistedventilation that takes into accountmost principles of proportionalventilation. Pressure is applied bythe ventilator in proportion to theelectrical activity of the diaphragm,recorded with a dedicatednasogastric tube with a multiplearray of electrodes placed in thedistal oesophageal portion [17].NAVA is now commerciallyavailable on the Maquet Servo i.

ATC

In order to reduce patient work ofbreathing arising from endotrachealtube resistance, some ventilators

(e.g. Dräger Evita 2 dura, 4 and XL,Covidien PB 840, Viasys Avea, GEEngstrom Carestation and allHamilton Medical ventilators) keeptracheal pressure constant duringinspiration and expiration. Theoutput delivered by the ventilatoradapts automatically andimmediately to the inspiratoryeffort of the patient. This control isnamed ATC. Recent studies haveshown that, in contrast to PSVwithout ATC, the latter is a suitablemode to compensate for theadditional work of breathing due tothe endotracheal tube in patientswith increased ventilatoryrequirements. They found thatwhen the additional work ofbreathing due to the endotrachealtube was compensated, noadditional level of PSV wasrequired [30, 31].

Dual modes

To generate further confusionamong buyers, the so-called “dualmodes of ventilation” (e.g. PRVC orVAPS) have recently beenintroduced in the critical care area.

The buyer must be aware that evenif the target of the dual mode is agiven tidal volume, this volume canbe reached by increasing flow or byincreasing pressure to the airway toreach the preset volume.

Another difference is that gasdelivery may be adjusted eitherwithin each breath (e.g. VAPS in theRespironics Esprit) or breath bybreath (e.g. PRVC in the MaquetServo i; Auto Flow in the DrägerEvita 2 dura, 4 and XL and Savinaand the Taema Extendl; VV+ in theCovidien PB 840; PCV-VG andBiLevel-VG in the GE EngstromCarestation, etc.) [6].

ASV

ASV may be thought of as an“electronic ventilator protocol”(defined as optimal control) [8] thatincorporates the most recent andsophisticated measurement toolsand algorithms in an attempt to

make ventilation safer, easier, andmore consistent [32, 33]. This modeis designed to accommodate notonly ventilated patients who arepassive, but also those who arebreathing actively. ASV recognisesspontaneous respiratory activityand automatically switches thepatient between mandatorypressure-controlled breaths andspontaneous pressure-supportedbreaths. With ASV, the cliniciandetermines the desired minuteventilation, and the algorithmdetermines the optimal respiratory-rate/tidal-volume combinationaccording to the patient’srespiratory mechanics. Any changein respiratory mechanics or patienteffort results in an updated optimalbreathing pattern, and ASVcontinuously and gently moves thepatient to the new, updated, target.Intelligent breath-to-breath safetyrules maintain ventilationparameters within safety ranges,and if for any reason the patientfails to breathe actively, ASVautomatically increases the numberof mandatory pressure-controlledbreaths that are needed in order tomaintain the minute volume target.The intrinsic requirement fordetermination of the optimalbreathing pattern is the breath-to-breath measurement of respiratorymechanics, including the expiratorytime constant, based on thevolume–flow loop method [34].


1. The buyer should determinewhich ventilatory modes will beused regularly in order not topurchase sophisticatedventilatory mode options thatwill seldom be needed. Thisshould result in cost savings.

2. The buyer must be aware of thelimitations of dual modes ofventilation such asPRVC/AutoFlow, etc.: when apatient’s ventilatory demandshows an increasing tidal volume,once the target tidal volume isachieved the patient is no longersupported.

BG-09 Ventilators:BG-09 Ventilators 20/8/08 23:13 4

3. Buyers must be aware that onlyPAV+ (Covidien PB 840) offersthe possibility of carrying outproportional ventilation using anoninvasive measurement ofelastance and resistance. Onlyone ventilator has NAVA.(Maquet Servo i).

4. ASV is provided only byHamilton Medical. In spite ofsome limitations, such as thepresence of severe leaks or fastchanges in respiratory drivecausing ventilator algorithmmalfunctioning, it is the currentauthors’ opinion that ASV mayhave an important role where thenumber of operators is limiteddue to cost restraints (e.g. inemerging countries).

5. Evidence is lacking that anyventilation mode provides betteroutcomes, though there aredifferent potential advantagessuggested by surrogatephysiological variables [1].

5. Control system

The control system controls orservo-controls output variables andactivates safety systems. It can workwith either a pressure or a flowfeedback. All category 1 and 2ventilators use a closed-loop controlthat maintains a constantventilatory output by using outputas a feedback signal that iscompared with the operator-setinput [8].

Most manufacturers use flowinside the ventilator as a feedbacksignal for safety reasons. In thoseventilators where the outputvariables are monitored at theairway opening (proximal airwaysensor), the feedback control isswitched to the proximaltransducer (e.g. Hamilton Medicalventilators) depending onventilatory mode. On the HamiltonGalileo and G5, during controlledmodes, the servo valve is driven byan internal flow sensor while theexternal flow sensor is used for

monitoring and safety. In theadaptive modes (e.g. ASV), theservo valve is driven by theproximal flow sensor while theinternal flow sensor is used tocheck values given by the proximalflow sensor (plausibility).

In ventilation modes dedicated topre-term babies, some category 1PPVs switch the control of outputvariables and their feedback systemto the proximal airway sensor (e.g.Dräger Evita 2 dura, 4 and XL,Maquet Servo i, GE EngstromCarestation). The Viasys Aveaallows the user to choose betweentwo types of proximalpneumotachograph (sensor): “hotwire” and “Osborne’s variableorifice”.

Recommendation to the buyer

1. The buyer must be aware of thedifferent places where thefeedback signal is recorded. Theadvantages and disadvantages ofdifferent locations of outputsensor are discussed in [6].

6. User interface

The control panel allows the user tointeract with the ventilator in order toset ventilator parameters and verifythrough monitoring that they arecorrectly applied. Some parametersare set directly whereas other arederived from measurements.

With the current generation ofPPVs, the user interface iscommonly a touch pad and/orrotary encoder with or without atouch-screen control. In most PPVs,a two-step process of changing andthen accepting a given parameter isused [6]. Some ventilators use anintelligent interface to decrease thedemand on the user's cognitiveresources (e.g. "dynamic lungs"Hamilton Medical G5).

Only two category 2 PPVs can beremoved from their conventionaluser interface (as docking stationsof a PC) to be used as transport

ventilators with their built-in userinterface (ResMed Elisée 350, ViasysPulmonetic LTV 1200).


1. User interfaces vary significantlyamong PPVs. Unfortunately thereis no consistency amongmanufacturers in terms oflabelling ventilator functions,particularly ventilation modes [6].

2. The large number of ventilatormodes, settings and monitoringcan result in systems that arepoorly designed, nonintuitiveand almost impossible toremember, with the subsequentneed for clinicians to refer tomanual ventilator instructions inorder to interpret poorly labelledcontrols or to navigate multiplelevels of software [3]. As aconsequence, the buyer must beaware that there will often be alearning curve to get a “feel” forfull navigation of the interface.

3. The buyer must choose theventilator according to currentneed (number of beds, emergencyroom area, different operatorsrotating in the shifts, etc.).

4. Last but not least, anotherpractical consideration is whetherthe monitor displaying thesettings, curves, alarms, etc., iseasily readable.

7. Safety and alarm systems

The ventilator safety system aims atavoiding any damage to the patientdue to ventilator malfunction.

In case of electrical failure, there is aroom-air inlet that will let thepatient breathe through asimultaneous opening of theinspiratory and expiratory valves.This may be of little or no help,however, in patients who aresedated and paralysed.

Another safety system is thepresence of an overpressure valve,positioned between the inspiratoryand expiratory valves, which can






09

unload any excess pressure in thecircuit. In most ventilators, it isusually set to open above a 100cmH2O threshold and thus it doesnot limit the risk of barotrauma.

On the contrary, the use ofmicroprocessor-controlledfunctions, with automated alarmresponses for events such asretrograde ventilation in case ofapnoea (back-up ventilation) duringassisted ventilation or pressure-targeted modes, or expiratory valveopening when a maximum tolerablepressure is reached, is moreeffective in reducing mechanicalventilation-related risks. Mostventilators can be set to start back-up ventilation in all ventilatorymodes in case of ventilatormalfunction.

Some ventilators (e.g. Dräger 2dura, 4 and XL, GE EngstromCarestation, Covidien PB 840,Viasys Avea) can differentiatebetween a leak and a realdisconnection, allowing theventilator to stop the highcontinuous flow produced in caseof an actual circuit disconnection(e.g. during patient suctioning). Thismay avoid environmental pollution.

The alarm system must alertmedical and paramedical personnelof an event that requires theirintervention [37]. These events canbe technical ones (e.g. related toventilator performance) or clinicalones, due to a change in patientconditions.

All ventilators in a critical care areashould have alarms for:

• electrical failure;

• compressed gas supply problems;

• patient disconnection from theventilator;

• changes in airway pressure (involume-targeted mode);

• changes in tidal volume (inpressure-targeted mode);

• changes in minute ventilation (involume- or pressure-targetedmodes); and

• changes in Fi,O2.

There are default values for alarmthresholds, beyond which anaudible alarm and/or a visualindicator will go off – but thesevalues should be set case by caseaccording to clinical situation,ventilation mode and kind ofinterface, and possibly modifiedwhenever these conditions change.


1. All PPVs for critical care exceptfor those designed only for NIV(Medisize Zephyros) and theRespironics Vision (this ventilatordoes not feature volume-targetedventilation) are life-supportdevices. It is imperative that theyrespond in a safe manner in caseof ventilator malfunction(pneumatic or electrical failure).

2. In spite of the safety system nowprovided in all PPVs, the buyermust be aware of the presence inthe hospital of electrical powergenerators in case of main powerblackout. Users must be informedof the units attached to theemergency generator.

8. Monitoring system

The monitoring system is not a partof the ventilator itself and itsabsence will not jeopardise properventilator functioning. However, itis of the utmost importance inoptimising ventilatory assistance.Some turbine-driven ventilators(e.g. ResMed Elisée 350, ViasysPulmonetic LTV 1200) can beconnected to a full-size screen toprovide additional monitoring.Thus, even though clinical andinstrumental monitoring based onpatient vital signs is very relevant,frequent control of both presetparameters on the ventilator andtheir actual application throughmachine-integrated systems iscrucial in the ICU setting.

In any ventilator, directlymeasurable variables are the

pressure applied to the airway andthe flow, while other parameterscan be derived from the analysis ofthese signals. Measured variablesand derived parameters are shownboth on a graphical display and asnumerical data in all ICU PPVs.Continuous display of airwaypressure, flow, and/or volumecurves is available with all currentICU ventilators.

Volume is usually calculatedthrough the integration of flow. Theflow sensors used in current PPVsare all reliable, but only those inHamilton Medical ventilatorsmeasure flow at the proximalairways in adult patients. Mostventilators measure flow inside theventilator, so the total calculatedvolume will include not only thedelivered volume, but also thevolume that is compressed insidethe ventilator circuit. To solve thisproblem, some ventilators ask foran initial operation to calculate tubecompliance (see section 9; estimatedat about 2–3 mL·cmH2O-1 in adultcircuits). However, the accuracy ofthe volume displayed variesconsiderably from that actuallymeasured at the ventilatory circuit“Y-piece”. The discrepancyincreases with increased tidalvolume [6]. Direct visualisation offlow–time and pressure–timewaveforms provides valuableinformation about the quality ofventilator–patient interaction (e.g.ineffective respiratory efforts,ventilator auto-triggering,dyssynchrony between aspontaneously breathing patientand the ventilator, dyssynchronydue to air leaks in PSV mode or toavoid inadequate inspiration orexpiration times during pressure-controlled ventilation) [36].

In totally mandatory breaths, mostPPVs are able to noninvasivelymeasure respiratory systemmechanics thanks to functions thatregulate inspiratory and expiratoryvalve closing [37].

Furthermore, most machines allowfurther display of pressure–volume




or flow–volume loops with thepossibility of performing a low-inflation pressure–volume curve(e.g. Taema Extend, GE EngstromCarestation, Viasys Avea).

Interestingly, few recent ventilatortypes incorporate the technology tomeasure an additional pressure,such as tracheal pressure oroesophageal pressure. The ViasysAvea and Hamilton Medical Galileoand G5 do offer this facility [38].

Although there is still a lack ofadequate easy-to-use technologies,this type of monitoring mayprovide useful information on therespiratory mechanics ofspontaneously breathing patients atthe bedside [39, 40].


1. The buyer should determinewhich monitoring capabilitieswill be used regularly in ordernot to purchase sophisticatedmonitoring systems that willseldom be needed. This shouldresult in cost savings [3].

2. In adult patients, there may beno significant differencesbetween the two sites ofmeasurement: distal (inside theventilator) or proximal (at theairway opening), except for abetter detection of start ofinspiration and end of

inspiratory effort. On the otherhand, during ventilation in pre-terms babies where patient flowis very low, a proximaltransducer is strongly advised.

3. Regarding the monitoring ofrespiratory mechanics,pressure–volume curves (low-inflation technique) are still apoint of discussion with regardto strategies for settingmechanical ventilation inpatients with acute respiratorydistress syndrome. However, itshould be kept in mind thatinterpreting the informationprovided by these curves anddrawing therapeutic conclusionsmay require some experience orat least clear strategies to dealwith the results.

4. Finally, it must be borne in mindthat the sensor accuracy of manyventilators lacks proper validation,and thus measured data can differ(even substantially) from realvalues [41].

9. Respiratory circuit andaccessories

The circuit is not a true part of thePPV and it is often sold separately.In spite of being a device positionedexternally to the PPV, its featuresmay change among differentventilators.

Double-limb respiratory circuit

The double-limb circuit iscomposed of an inspiratory and anexpiratory limb whose proximalends are connected to theinspiratory and expiratory ports,while the distal parts are connectedto the so-called Y-piece. The Y-piececan be the connected to the cathetermount with or without a proximalsensor (fig. 22).

In some PPVs (e.g. HamiltonMedical ventilators) thepneumotachograph is positioned atthe Y-piece (sensor at the proximalairways; fig. 22). Flow and pressuremeasured at this site can be usedeither as simple monitoring tools orto control some ventilator functions(e.g. Hamilton Medical ventilators,Dräger Evita 2 dura, 4 and XL,Maquet Servo i, GE EngstromCarestation and Viasys Avea whenusing pre-term ventilation).

The effective compliance of thepatient circuit is a combination oftubing compliance and gascompressibility [10]. Most high-pressure ICU PPVs provideautomatic compensation of circuitcompliance according to theircircuit place (adult versuspaediatric). However someventilators still lack this automaticcompensation (e.g. Dräger Savina,Covidien Supportair)

Figure 22. (left to right). Hamilton MedicalG5, Raphael and C2. All use a sensorpositioned at the airway opening at the Y-piece before the cathether mount (red arrow).

Figure 23. Dräger Evita 4/XL with itssensor positioned at the airway openingduring NeoFlow ventilation.


When the flow sensor is positionedproximal to the airways (e.g.Hamilton Medical ventilators,Dräger Evita 2 dura 4 and XL,Maquet Servo i and Viasys Aveawhen using pre-term ventilation)there is no need to calculatecompliance (fig. 23).

Single-limb circuit

The single-limb circuit is found onlyin category 2 PPVs, except for somecircuits that may have theirinspiratory and expiratory limbsembedded coaxially. Single-limbcircuits have no Y-piece and thecircuit is directly attached to thecatheter mount. This circuit isusually limited to anaesthesia use.Apart from coaxial circuits, threetypes of single-limb circuit can bedefined (figs 24 and 25).

i. Single circuit with an activenonrebreathing expiratory valve(e.g. a mushroom valve driven by

ventilator pressure such as in theCovidien Supportair). This valve ispositioned proximally to the patientclose to the catheter mount (fig. 24).The function of this valve is oftenan on–off function, which may alsowork as a PEEP valve. Althoughthey always avoid rebreathing, thiskind of valve may prolong theexpiratory time constant [42]. Thedead space between the proximalvalve and patient interface (e.g. themask) may affect total dead space.

ii. Single circuit without an activenonrebreathing valve. This type ofcircuit is found only on five PPVs(Respironics Vision, Dräger Carina,Covidien Supportair and RaphaelXTC and C2). It is also called anintentional leak circuit (fig. 25). CO2

is vented out through threemodalities, as shown in table 3.

In the first two types of intentionalleak circuit in table 3, the amount ofrebreathing always increases when

patient expiratory flow exceedsclearance of the vent system. Table 4shows the causes of CO2 rebreathingin an intentional leak circuit.

It was previously demonstrated thatan EPAP of 8 cmH2O was able toavoid CO2 rebreathing completely[43–46]. The use of a RespironicsPlateau Valve nonrebreathing valvemay completely avoid CO2

rebreathing in the intentional leakcircuit of a dedicated Respironicsventilator (e.g. Respironics Vision) [43].


1. Coaxial circuits should not beused in a critical care setting



09

Figure 24. A single-limb circuit with a mushroom expiratory valve (red arrows). The valve isdriven by the pressure coming from the pipe (green arrow).

Figure 25. A single-limb intentional-leakcircuit. The intentional leak is providedthrough a connector with “vent slots”(Whisper) or a “Plateau Valve” positionedin series in the circuit close to the interface(a and d), through “vent holes” in the mask(c) or through a vent hole positioned in thecircuit (b). The red arrows indicate where airflows through the leak.

Venting system build-up in the interface (vent slots or holes) or in thecircuit proximally to the airway

Venting connector (e.g. Whisper swivel by Respironics)

Plateau Valve#

#: This valve is a reusable exhalation device that provides a continuous leak path in the patientcircuit when used with Respironics continuous positive airway pressure (CPAP) and bilevelsystems. Although it is not an active valve it does not allow CO2 rebreathing [45].

Table 3 Modalities of CO2 venting in intentional leak circuits

Patient’s expiratory flow#

Ventilator expiratory flow output

Level of PEEP/EPAP

Position of vents¶ in the circuit aswell as in the mask and maskdead space

PEEP: positive end-expiratory pressure;EPAP: expiratory positive airway pressure.#: In turbine-driven ventilators featuring aproportional valve to control ventilatoryoutput (e.g. Respironics Vision) patientexpiratory flow does not affect rebreathing;¶: Manufacturers must always specify “ventsystem” features.

Table 4 Causes of CO2 rebreathing in anintenional leak circuit




HR adv 105+205 21x28cm.indd 1 07-07-2008 16:34:01

because of their increasedrespiratory resistance due todecreased expiratory limb size.

2. The lack of a Y-piece (e.g. coaxialcircuit, intentional leak circuit)does not allow a spacer to bepositioned on the distal part ofthe expiratory limb to deliverdrugs using a metered doseinhaler (MDI). For the samereason, conventional double-limbcircuits should always bedetachable at the Y-piece in orderto install an in-line spacer forMDI use or use of an activenebuliser [47].

2. Buyers must be aware ofrespiratory circuit complianceduring mechanical ventilation. Inadult patients, all category 1ventilators ask to measure circuitcompliance before beginningmechanical ventilation bymeasuring flow and pressure.Hamilton Medical ventilatorsmeasure pressure at the airwayopening and do not need tomeasure respiratory circuitcompliance.

3. While in adult patients there maybe no significant differencebetween using two types ofcircuit with different compliances(e.g. 2 mL·cmH2O-1), in patientswith low tidal volume (e.g. 50 mL;e.g. paediatric or neonates), circuitcompliance plays a major role(patient compliance and circuitcompliance are connected inparallel). Ventilators featuringpre-term ventilation (e.g. DrägerEvita 2 Dura, 4 and XL, MaquetServo i, GE Engstrom Carestation,Viasys Avea, Hamilton MedicalGalileo and G5) use a sensor atthe airway opening.

4. When automatic compliancecompensation is not present, itmust be calculated as describedelsewhere [8].

5. When using a ventilator with anintentional leak circuit (notprovided with an activeexpiratory valve) an anti-suffocation valve is strongly

advised in situations where thepatient may be left withoutcontinuous nursing care (e.g. apatient switched from critical careto the general ward).

Accessories

High-pressure hoses and connectinghardware

Hoses may have different quickconnections to interface withcentralised high-pressure gas or gascylinders. This may also hold truefor electrical power connections.

Respiratory circuit

See section 9 above.

Heated humidifiers

Heated humidifiers are often soldseparately. It is beyond the scope ofthe present guide to discuss theutility of active or passivehumidification in critical care.

Carts, brackets and carriers

Carts may be a simple basemounted on wheels or a complexstorage facility. Brackets areprovided in order to mount thePPVs at the patient’s bedside or ona pneumatic column. Carriers allowgas tank and external batteryconnection, facilitating patienttransport.

External batteries and externalcompressors


Filters

PPVs may be equipped withinspiratory as well as expiratoryfilters. The former may protectpatients from particulates in thepiped or compressed gas supply,while the latter may protectdownstream sensors andcomponents from particulatescoming from aerosolised drugs. Thefilter may have an impact on flow,by adding resistance. Filters also

increase in resistance over time.Filters can be disposable orreusable.

Nebulisers

Some ventilators are capable ofpowering their own nebuliserwithout affecting the deliveredFi,O2. This increases their capabilityto deliver a large volume of aerosolin the therapeutic range (e.g.Maquet Servo i, eVent InspirationLS, Dräger Evita 2 dura, 4 and XLand Savina) [6].


1. High-pressure hoses andconnecting hardware must becompatible with the local gas andelectrical plugs.

2. According to team needs, carts,brackets or carriers to facilitatepatient transport should bepurchased along with theventilator.

3. In spite of the fact that manyventilators now feature their ownnebuliser, drug aerosols withinthe expiratory circuit may causeexpiratory sensor malfunction.Expiratory limb filters may beused to prevent sensor damagebut may have a greater impact onexpiratory resistances and onoperating costs [6].

Special needs

Pre-term ventilation


Heliox

See section 1 above (gas blending).

Lung-sigh, recruitment andprotective lung software

SmartCare is a mode of ventilationspecifically designed to expedite theweaning process. It utilises onlypressure-support breaths, withvarying levels of inspiratory


pressure. The principle [48] uses themeasured respiratory rate to adjustthe level of PSV (increasing PSV ifrespiratory rate is high, decreasingPSV if respiratory rate is low).

SmartCare analyses ventilationevery 2 min: more often than anytherapist could. The objective ofSmartCare is to obtain a target levelof pressure support that is toleratedwell and which eventually indicatesthe patient’s readiness to bediscontinued from the ventilator.The level of pressure support is alsoadjusted to maintain tidal volumeabove, and end-tidal carbon dioxidepressure below, certain values. Theswitch from controlled ventilationto SmartCare is not automatic, andrequires manual intervention. Thissoftware option is now available onthe Dräger Evita XL.

The sigh breath is a deliberatelyincreased tidal volume for one ormore breaths at regular intervals[49]. The lack of a real evidencebase about sighs means that fewcurrent-generation PPVs featurethis function (e.g. Dräger Evita 2dura, 4 and XL, Taema Extend).Only one ventilator allows realmanipulation of the sigh function:the eVent Inspiration LS equippedwith the Smart Sigh function. Usersmay define volume- or pressure-based sighs, their frequency andmultiples as well as their amplitude(+0–50% of tidal volume or pressuresetting). Three recent clinicalstudies, however, havedemonstrated improvedoxygenation with the use of sighs asa recruitment manoeuvre inpatients with acute respiratoryfailure [50–52]. Some ventilatorshave dedicated software for “openlung” and “lung protectivestrategy” (e.g. Lung ProtectionPackage (LPP) for Dräger Evita XLand Open Lung Tool (OLT) forMaquet Servo i).


1. In spite of the lack of a realevidence base about sighs, thereis now an increased interest in the

sigh option as well as dedicatedsoftware for lung recruitment andopen lung techniques.

2. SmartCare in EvitaXL (as asoftware option) is a mode ofventilation designed to expediteweaning. Patient readiness andability to wean also need to beassessed by the clinician.

3. As previously mentioned,evidence is lacking that anyventilation mode or ventilatorsoftware package can providebetter outcomes [1].

Dedicated NIV ventilatorsand ventilator software forNIV

Modes of ventilation

The results of a recent bench-modelstudy confirm that leaks interferewith several key functions of ICUventilators [53]. On most category 1ventilators, conventional softwarefor invasive ventilation leaks led toan increase in trigger delay andworkload, a decrease inpressurisation and delayed cycling[53]. Air leaks during NIV are anobstacle to correct expiratorysynchronisation. In fact, anyadditional flow due to leakage willdelay or even prevent reaching thethreshold. When this happens, theexpiratory trigger is unable tosynchronise its action with the endof patient inspiratory effort. Onsome ventilators, as mentionedpreviously, the threshold of theexpiratory trigger can bemanipulated, thus allowingimproved expiratorysynchronisation [19–23]. The abilityto set a maximum inspiratory timebeyond which there will always bea mandatory cycling, or elseparticular algorithms, can alsogreatly reduce problems related toexpiratory I/E cycling during PSV[24]. Most manufacturers have nowdeveloped microprocessor-controlled ventilators with differentalgorithms that provide bothvolume- and pressure-limited

modes with special featuresdesigned to facilitate NIV delivery.

When using noninvasivecontinuous positive airway pressure(CPAP), most ventilators imposeonly a small effort to trigger, butmost also provide low-levelpressure support and impose anexpiratory workload [54, 55].Pressure triggering during CPAPdoes require a slightly greaterpressure than flow triggering [56]although flow triggering gavebetter results in NIV-–PSV mode inchronic obstructive pulmonarydisease patients. However, even the“best” triggering system may bemarkedly affected by the interfaceused (e.g. helmet or other large-volume interface).

Category 2 ventilators with anintentional leak circuit and bilevelventilation (e.g. BiPAP byRespironics Vision) have beentested in many clinical and benchstudies. BiPAP (IPAP/EPAP) is apressure-limited flow or time-cycled mode of ventilation. Whenthe patient is active, BiPAP is, insimple terms, a PSV working in thebelow PEEP setting. It has a veryefficient inspiratory/expiratorytrigger algorithm calledAutoTrakTM. Many so called otherbilevel ventilators (e.g. CovidienSupportair) use the below PEEPsetting.

The Respironics Vision also featuresNIV PAV [57–58].

Circuits

Both single- or double-limb circuitsare provided in NIV ventilators.Single-limb circuits without anactive nonrebreathing valve arefound on only three category 2PPVs (Respironics Vision, DrägerCarina, Covidien Supportair). TheDräger Carina and CovidienSupportair also have the option touse a single-limb circuit with anactive mushroom valve (figs 17 and24). One category 1 ventilatormanufactured only for NIV(Medisize Zephyros) has a



09


double-limb circuit, like all othercategory 1 ventilators

Alarms

During NIV it is also advisable tohave alarms for significant air leaks.These are characterised by areduction in airway pressure andexpiratory tidal volume when involume-limited mode, and a risinginspiratory tidal volume anddecreasing expiratory tidal volumein pressure-limited mode.

Monitoring

When using NIV, inspiratory tidalvolume measurements can beconfounding, because the totalvolume provided by the ventilatorequals the sum of volume actuallysent to the patient plus leakvolume. Thus expiratory tidalvolume is more representative ofalveolar ventilation than isinspiratory tidal volume. Withventilators that measureinspiratory and expiratory tidalvolumes separately, leakmagnitude can be estimated bycalculating their difference. This is

true for ventilators that measureboth inspiratory and expiratoryflow inside the ventilators (e.g.Medisize Zephyros, RespironicsEsprit and all category 1ventilators) and ventilators thatmeasure flow proximally to theairway (e.g. Viasys PulmoneticLTC 1200, VersaMed iVent 201 ICand all Hamilton Medicalventilators).

Ventilators designed solely for NIV

Only one ventilator (the MedisizeZephyros) is designed solely forNIV. The Respironics Vision andDräger Carina, although designedmainly for NIV, can be also used inthe critical care area for invasiveventilation. The Medisize Zephyroscan deliver NIV with the abovePEEP setting (PSV) The ventilatorhas software to set up NIV withdifferent interfaces (e.g. mask orhelmet).


1. Overall, NIV modes can partiallyor completely correct drawbacksdue to air leaks, but there are

wide variations betweenmachines in terms of efficiency[53]. Clinicians should be awareof these differences whenapplying NIV with an ICUventilator.

2. NIV CPAP delivered byventilators is reliable. However,when using only genuine CPAP,continuous-flow systems havevery good performance and arecheaper [52]. CPAP should not beused in ventilators with a doublecircuit when using a helmetinterface [59–61].

Conclusions

Purchasing a PPV should be basedon a basic knowledge of machine-specific functions and anunderstanding of the physiologicalrationale. It is therefore crucial tounderstand how these functionsshould interact with the operativeenvironment and patients’ needs.Once this goal is achieved, aventilator may be chosen based onspecific needs, constraints andcosts. ■



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